Ventricular arrhythmias include ventricular tachycardia (VT) and ventricular fibrillation (VF). A tachycardia is a fast heart rate (usually over 100 beats per minute) typically caused by disease or injury. It can also be part of a normal response to increased activity or oxygen demands. The average heart beats between 60 and 100 times per minute. When the tachycardia is due to disease or injury, it usually requires treatment. Tachycardias may begin in the upper chambers of the heart (the atria) or the lower chambers of the heart (the ventricles). A ventricular tachycardia (VT) begins in the ventricles. Some are harmless, but others are life threatening in that they can quickly deteriorate to a ventricular fibrillation.
A ventricular fibrillation (VF) is a very fast, chaotic heart rate (usually over 300 beats per minute) in the lower chambers of the heart, resulting from multiple areas of the ventricles attempting to control the heart's rhythm. VF can occur spontaneously (generally caused by heart disease) or when VT has persisted too long. When the ventricles fibrillate, they do not contract normally, so they cannot effectively pump blood. The instant VF begins, effective blood pumping stops. VF quickly becomes more erratic, resulting in sudden cardiac arrest. This arrhythmia must be corrected immediately via a shock from an external defibrillator or an implantable cardioverter defibrillator (ICD). The defibrillator stops the chaotic electrical activity and restores normal heart rhythm.
Appropriate detection of ventricular arrhythmias (requiring high voltage therapy) and discrimination of supraventricular arrhythmias (not requiring high voltage therapy) in a noisy signal is challenging and of great importance, particularly as it relates to an implanted defibrillator using subcutaneous extracardiac electrodes or an external defibrillator. Since such signals are composed of both cardiac and skeletal myopotentials, motion artifacts, electromagnetic interference (EMI), etc., appropriate detection of arrhythmias relies heavily on accurate detection of each subcomponent of such composite signals. There is a need for improved methods and systems that can detect ventricular arrhythmias and discriminate between the different types of arrhythmias even when myopotentials and motion artifacts affect that cardiac signal that is being monitored.
Sensing cardiac electrical activity from electrodes spatially removed from the heart, as in a surface ECG or in a subcutaneous extracardiac configuration is challenging. Non-cardiac signals such as skeletal myopotential and motion artifact can easily be mistaken for an arrhythmia, which can lead to inappropriate therapy. Given the composite nature of subcutaneous signals, it is useful that a scheme for detecting non-cardiac events allow for changes (e.g., daily or hourly) in signal characteristics.